1. Field of the Invention
This invention relates to the polymerization of olefins. More particularly, this invention relates to a process having catalyst compositions which are useful for polymerizing one or more monomers comprising ethylene to polymers having a narrow molelcular weight distribution and a good balance of physical properties.
2. Description of the Prior Art
It is known that catalysts of the type variously described as coordination, Ziegler, Zieger-type, or Ziegler-Natta catalysts are useful for the polymerization of olefins under moderate conditions of temperature and pressure. It is also known that the properties of the polymers obtainable by the use of such catalysts, as well as the relative economies of the processes used to prepare the polymers, vary with several factors, including the choice of the particular monomers, catalysts components, polymerization adjuvants, and other polymerization conditions employed.
During the years since Ziegler catalysts were first publicly disclosed, there has been a considerable amount of research conducted on the use of such catalysts; and numerous publications have resulted from that research. These publications have added much to the knowledge of how to make various types of olefin polymers by various types of processes. However, as is apparent from the amount of research on Ziegler catalysts that is still being conducted throughout the world, as well as the number of patents that are still being issued to the inventors working in the field of Ziegler catalysts, the means of attaining certain results when polymerizing olefins with Ziegler catalysts are still frequently unpredictable. The fact that this situation exists is sometimes due to the need to obtain a previously-unattainable combination of results; occasionally due to difficulties in obtaining the same results in a commercial-scale apparatus as in a laboratory-scale reactor; and often due to a polymerization parameter's having an effect, or side-effect, in a given type of polymerization process that is different from effects achieved by its use in prior art processes of a different type.
One aspect of Ziegler catalysts in which the need for further research has been found to exist has been in the field of preparing ethylene polymers having a narrow molecular weight distribution and a good balance of physical properties. Such polymers have particular application in the production of articles that are formed by injection molding; typically have molecular weight distributions such that their normalized V.sub.30 /V.sub.300 melt viscosity ratios are in the range of about 1.5 to 2.3, with the ratios in the lower portion of this range being generally preferred but difficult to attain with known processes that might otherwise be commercially feasible; and--like other polymers intended for commercial use--are desirably prepared by a process which is as economical as possible as well as being capable of producing a polymer having the desired properties.
There are, of course, known processes for preparing injection molding resins by polymerizing ethylene with the aid of Ziegler catalysts. However, the known processes typically suffer one or more of the disadvantages of lack of economy, inability to produce polymers having a suitable balance of properties, and/or unrealiability in producing such polymers-particularly in commercial-scale operations. U.S. Pat. No. 4,003,712 by Miller teaches a gas-phase fluidized bed system and process which are capable of being scaled up to commercial size and, being solvent-free, would be less expensive than processes which use solvents or liquid diluents. However, Miller's silyl chromate catalysts do not give polymers of the desired narrow molecular weight distribution and good balance of physical properties. His system contains some features which tend to shorten commercial "on-stream" time. He does not teach how to avoid polymer build up on reactor surfaces, a phenonomenon variously referred to as "coating", "fouling", or "sheeting".
What is still needed is a process employing a catalyst which (a) is suitable for use in a gas-phase polymerization process, (b) is capable of yielding polymers having a narrow molecular weight distribution and a good balance of physical properties, (c) has sufficient activity to be economically attractive, and (d) does not cause reactor wall fouling, and (e) a gas-phase fluidized bed process which allows the catalyst to perform at its full potential at commercial scale.
British Pat. No. 1,489,410 (Monsanto) teaches gas-phase polymerization processes which, because of their use of supported Ziegler catalysts having a vanadium component and other factors, are commercially attractive processes. However, as taught in the patent, the processes are designed to result in the formation of polymers having the broad molecular weight distributions suitable for blow molding resins rather than the narrower molecular weight distributions needed for injection molding resins; and the patent itself does not suggest how its processes might be modified to result in the formation of polymers having narrower molecular weight distribution. Attempts to make the processes of the patent suitable for the preparation of injection molding resins by combining its teachings with the teachings of publications that discuss means of narrowing molecular weight distribution have not been successful. For example, polymers having a sufficiently narrow molecular weight distribution have not been obtained when Monsanto's preferred vanadium halides have been replaced with the alkoxy group-containing vanadium compounds which are within the scope of their patent and which U.S. Pat. Nos. 3,457,244 (Fukuda et al.) and 3,655,583 (Yamamoto et al.) teach to result in the production of polymers having narrower molecular weight distributions when unsupported catalyst systems are employed.
U.S. Pat. No. 2,965,626 by Pilar et al discloses polymerizing organic compounds containing ethylenic unsaturation under relatively mild polymerization conditions with catalysts and alcohol catalyst promoters. More specifically, Pilar et al found that the polymerization activity of the catalyst prepared by reaction of alkali reagents with the specified metal salts can be substantially increased by the inclusion of an alcohol in the reaction zone. U.S. Pat. No. 3,163,611 by Andersen et al. pertains to the production of high density polyethylene by polymerizing ethylene in the presence of a catalyst exemplified by the material obtained by the interaction of a trialkylaluminum with titanium tetrachloride. U.S. Pat. No. 3,202,645 to Yancey presents a process for polymerizing and copolymerizing alpha mono and di-olefins by a catalyst comprising (a) the product of the reaction between a compound of a metal chosen from the group consisting of the metals of Groups IIb and IIIb (where the group numbers correspond to the Mendeleev Periodic Table) and hydroxyl groups on the surface of a finely-divided particulate inorganic solid, preferably finely-divided silica or alumina, and (b) a halide-type compound of a Group IVa, V, VIa, VIIa, or period 4 of Group VIII metal. The polymerization or copolymerization reaction can be effected at suitable temperatures within the range of from about -25.degree. C. to about 250.degree. C., and pressures ranging from below atmospheric upwardly to any desired maximum pressure, for example, 30,000 p.s.i.g. or even higher pressures. U.S. Pat. No. 3,219,652 by Hill et al discloses an additive modified Ziegler-type catalyst comprising the combination of an organometallic compound and a transition metal halide of a conventional Ziegler-type catalyst and a polyether or hydroxyether. U.S. Pat. No. 3,718,636 to Stevens et al teaches obtaining polyolefins having a wide distribution of molecular weights through the use of catalysts comprising an organometallic compound and a solid complex component obtained by reacting a solid bivalent metal compound with an impregnation agent which consists of an organometallic compound, separating the solid reaction product, and reacting the solid reaction product with a halogenated derivative of a transition metal. Stevens et al teaches in U.S. Pat. No. 3,787,384 another catalyst suitable for use in olefin polymerization and olefin copolymerization which comprises
(a) at least one organometallic compound, and
(b) a solid catalytic component obtained by reacting a support compound of silica, alumina or both silica and alumina with a compound of the formula MR.sub.n X.sub.m-n in which M is aluminum or magnesium, R is a hydrocarbon radical containing 1 to 20 carbon atoms, X is hydrogen or a halogen, m is the valence of M, and n is a whole number not less than 1 nor greater than m, separating the solid product of the reaction, reacting said product with an excess of a halogen-containing transition metal compound, and separating the solid reaction product. U.S. Pat. No. 3,925,338 to Ort teaches that control of particle size of olefin polymers produced by gas-phase polymerization of at least one olefin using Ziegler-type catalysts deposited on solid supports in a fluidized-solids operation is effected by controlling the particle size of the catalyst support. U.S. Pat. No. 4,232,140 also to Ort discloses the use of trichlorofluoromethane as a promoter in the polymerization and copolymerization of the ethylene with supported Ziegler-type vanadium compound/alkylaluminum compound catalysts in the presence of hydrogen. Ort finds that polymer yields with his supported vanadium-based catalysts are too low for commercial viability unless the catalyst is promoted to high yield with the trichlorofluoromethane promoter. The viscosity ratio data in Ort's examples, which may be related to molecular weight distribution, indicate that none of the polymers have narrow molecular weight distribution. Ort does not teach or suggest how to avoid reactor fouling.
Fukuda et al in U.S. Pat. No. 3,457,244 also teach that ethylene copolymers or terpolymers having narrow molecular weight distribution can be obtained by the use of an unsupported catalyst composition prepared by (1) mixing an alcohol containing 1 to 12 carbon atoms with VOCl.sub.3 and then (2) mixing the mixture thus obtained with an alkylaluminum compound in the presence of the monomers to be interpolymerized, and there are other patents, e.g. Stamicarbon's British Pat. No. 1,175,593 and U.S. Pat. Nos. 3,535,269 (Tanaka et al.), 4,071,674 (Kashiwa et al.), and 4,256,865 (Hyde et al.) which teach the use of catalyst compositions prepared by adding an alcohol at some stage during the catalyst preparation. However, although some of these patents are concerned with the production of polymers having narrow molecular weight distribution, none of the prior art teaches or suggest an economical, gas-phase, fluidized bed process and catalyst for commercially producing ethylene polymers of narrow molecular weight distribution and a good balance of physical properties.